comm - intercommunicator (handle)
high - order parameter (boolean)
out - the resulting intracommunicator (handle).
The MPI_Intercomm_merge() function is used to merge the local and remote groups within an intercommunicator comm into a single group. That new group is used to create an intracommunicator out . The value of high is used to order the ranks in the new intracommunicator. All processes in the same group should provide the same value of this parameter. If all processes in one group provide a value of high = false and all processes in the other group provide high = true then the union orders all process in the "low" group before the "high" group.
The C++ version of this function passes the new intracommunicator as the return value.
Note that the communicator must be a valid one (not MPI_COMM_NULL). PMPI_Intercomm_merge() is the profiling version of this function.
All MPI routines except for MPI_Wtime and MPI_Wtick return an error code. The the current MPI error handler is invoked if the return value is not MPI_SUCCESS. The default error handler aborts, but this may be changed with by using the MPI_Errhandler_set() function. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead. Note that MPI does not guarentee that an MPI program can continue past an error. In this implementation, all errors except MPI_ERR_INTERN or MPI_ERR_OTHER should always be recoverable.
In C, the error code is passed as the return value. In FORTRAN, all functions have an parameter ierr which returns the error code. MPI C++ functions do not directly return an error code. However, C++ users may want to use the MPI::ERRORS_THROW_EXCEPTIONS handler. This will throw an MPI::Exception with the corresponding error code. To prevent exceptions from being raised from within C and Fortran code, they will see all error return values as MPI_ERR_PENDING when this handler is chosen. In this implementation, call MPI::throw_exception() to throw the correct exception if this occurs.
MPI_SUCCESS - No error;
MPI_ERR_PENDING - Pending exception;
MPI_ERR_COMM - Invalid communicator;
MPI_ERR_ARG - Invalid output pointer, high parameter not 0 or 1, or high varies in a group;
MPI_ERR_INTERN - Out of memory. This may be fatal;
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